Synchronize local MySQL databases with a cloud database

Plan A: Galera cluster (as found in MariaDB) that includes the 3 servers.

Plan B: "Multi-source replication" wherein your two physical servers are Masters and the Cloud server is the Slave. Again, the requires MariaDB. (See DBHash's Answer.)

You can define FEDERATED tables in your "cloud" database: any queries on these tables will be transmitted from the "cloud" server to the relevant London/Dublin server over the MySQL client protocol (note that data is not copied to the "cloud" server, so it does not provide any form of backup service):

CREATE SERVER london FOREIGN DATA WRAPPER mysql OPTIONS (
  HOST 'london.mysql.example.com',
  PORT 9306,
  USER 'cloud_db_user',
  PASSWORD '...',
  DATABASE 'my_database'
);

CREATE SERVER dublin FOREIGN DATA WRAPPER mysql OPTIONS (
  HOST 'dublin.mysql.example.com',
  PORT 9306,
  USER 'cloud_db_user',
  PASSWORD '...',
  DATABASE 'my_database'
);

CREATE TABLE london_table (
    -- table definition as normal
)
ENGINE=FEDERATED
CONNECTION='london/original_table';

CREATE TABLE dublin_table (
    -- table definition as normal
)
ENGINE=FEDERATED
CONNECTION='dublin/original_table';

You could then define a VIEW that comprises the UNION of those federated tables. Unfortunately however, UNION views are neither insertable nor updateable—so if you need to commit any changes to the data you'd have to operate on the underlying (federated) table:

CREATE VIEW combined AS
  SELECT * FROM london_table
UNION ALL
  SELECT * FROM dublin_table;

You can set up MariaDB in cloud server and have LONDON and DUBLIN servers as masters. Multi master replication is available in MariaDB. I assume that the masters have different database names.

https://mariadb.com/kb/en/mariadb/multi-source-replication/

Presto is useful here as well.

you can deploy presto on the cloud machine, using mysql connectors connect to different geo databases, create different catalog schemas.

write queries to combine result from both database. something like:

select * from dublin.A
union all
select * from london.A

Some Random Links on this

Have two webservices for each database location. The webservices queries the respective databases on a periodical interval and insert the data into cloud database.